It is common to use solid phase extraction to remove dissolved species from a fluid, whether for analytic (small-scale) or purification (large-scale) purposes by means of a particulate-loaded porous web. Webs useful for such purposes include fibrillated polymeric webs (e.g., polytetrafluoroethylene (PTFE)), nonwoven polymeric webs (e.g., polyolefins, such as polyethylene and polypropylene), and wet-laid or polymer paper webs (e.g., webs prepared from fibrous pulp, such as cellulose, glass, aramid, or polyethylene fibers). Further, the particulate that is used in the particulate-loaded porous webs can be sorptive, reactive, or a combination of the two. Materials commonly used in these webs include, for example, activated charcoal, silica, alumina, silica coated by organic and inorganic materials, and poly(styrene-co-divinyl benzene).
However, when webs of the type described above are used in extractive processes where the carrier fluid comprises both dissolved species and suspended matter, pores in the webs may be quickly plugged with suspended matter, thereby requiring increased pressure to move the carrier fluid through the extractive process and decreasing the effectiveness of the web for removing dissolved species from the fluid. This plugging of web pores and resulting decrease in capability of the web for removing the dissolved species is referred to as web fouling. Examples of the fluids used in these extractive processes include blood or other physiological fluids, environmental fluids such as lake or stream water, and fruit juices containing pulp or other suspended matter. Methods that have been used to minimize web fouling include the use of glass bead prefilters (e.g., U.S. Pat. No. 5,366,632) and gradient density prefilters (e.g., U.S. Pat. No. 5,472,600) to prefilter suspended matter from the carrier fluid before it reaches the particulate-loaded porous web. Glass bead prefilters are superior in performance to traditional prefilter materials, such as diatomaceous earth, sand, cellulose or perlite expanded amorphous volcanic rock, in providing faster filtration times and decreased pressure drop. The beads are said to gain advantage due to features such a their spherical shape and inertness. Gradient density prefilters provide advantages due their capacity to separate a large amount and variety of suspended or interfering matter before an analyte comes in contact with the solid phase medium. In spite of these advantages, both types of prefilters can themselves become clogged, decreasing the efficiency of the extraction process. In large-scale industrial separation processes that require longer production run-times for each separation media change than are allowed by either of these prefilter means the separation processes are often limited by the length of time the prefilter means remain unclogged.